1. Field of the Invention
The present invention relates generally to the field of fiber optical technology, and in particular to systems and techniques for compensating for the thermo-optic effect in active optical fibers.
2. Background Art
Generally speaking, in a fiber laser operating at a high power level, it is desirable for the active gain fiber to be single-moded. Multimoded gain fibers pose a multitude of problems, including multimoded lasing, temporal instability, beam quality degradation, modal instability, and the like. Any of these issues can have catastrophic consequences in high-power fiber lasers and amplifiers.
Fibers utilized for high-power applications typically cannot be reliably made truly single-moded using conventional designs, owing to the low index contrasts that are required, which can also result in enhanced sensitivity to bend loss. As a practical matter, this issue can be overcome by utilizing multimode fiber designs having low loss for the fundamental mode and high loss for higher-order modes (HOMs). Such fiber designs are configured to provide strong coupling between the HOMs and the cladding modes, while minimizing coupling between the fundamental mode and the cladding modes. This arrangement causes the HOMs to leak into the cladding, while preserving the fundamental mode. The desired mode coupling characteristics are achieved through additional index features or physical effects, such as fiber coiling. Care must be taken to ensure that the structures used to enhance HOM loss do not increase loss of the fundamental mode to an unacceptable level.
However, the loss characteristics for the above-described fiber designs are adversely affected by the thermo-optic effect. In a fiber laser or amplifier, there is continuous generation of heat in the core due to the gain process (quantum defect) which is dissipated to the outside of the fiber through the cladding. This generation and dissipation of heat result in a temperature gradient in the fiber. This temperature gradient results in a corresponding index gradient, which can reduce the coupling of the HOMs to the cladding modes, thereby degrading the HOM suppression required for single-moded operation.
Filter fibers are another class of fibers which utilize similar ideas of coupling to the cladding to create enhanced loss beyond a specific wavelength while preserving low loss at the operating wavelength. Active filter fibers have been utilized to suppress nonlinearities like stimulated Raman scattering in high power lasers and amplifiers. The index gradient as a result of thermo-optic effect creates similar issues of position-dependent degradation of wavelength selectivity. To preserve the wavelength selectivity, compensation must be provided for this degradation.